The intensity of radio waves is poor in underground areas, back of buildings, and boundary areas of related mobile phone base stations having call areas of several hundreds of meters to more than ten kilometers in radius. One problem occurred in these areas is that it is difficult to connect to the base stations. One system provided in order to solve this problem is a micro base station having a call area of about several tens of meters in radius where phone calls can be made even in those areas. Such a micro base station is called a femtocell. In the following description, a system in which a femtocell is arranged is called a femto system.
FIG. 18 shows an example of a femto system. A femto system 10 includes a femtocell 30, a gateway apparatus 50, a media gateway 52, and a line switching unit 54. While the media gateway 52 may be included in the line switching unit 54, the media gateway 52 and the line switching unit 54 will be separately shown and described for the sake of clarification.
In the following description, the femtocell, the gateway apparatus, the media gateway, and the line switching unit are denoted by a Home Node B (HNB), a Home Node B Gateway (HNB-GW), a Media Gateway (MGW), and a Mobile-service Switching Center (MSC). Further, each terminal apparatus 20 connected to the HNB 30 is denoted by a User Equipment (UE).
The gateway apparatus 50 relays transmission and reception of voice data between the MGW 52 and the HNB 30, and is connected to the HNB 30 through a network 40. The voice data in the phone call is transmitted to the UE 20 through the MGW 52, the HNB-GW 50, the network 40, and the HNB 30 in this order, or transmitted from the UE 20 through the HNB 30, the network 40, the HNB-GW 50, and the MGW 52 in this order. Further, as described in a non-patent literature 6 and a non-patent literature 1, “The RUA has the following functions: Transparent transfer of RANAP messages”, “Iu UP is terminated in CN and HNB (i.e. not in the HNB GW), which means that, in the HNB-GW 50, data whose level is higher than Iu-UP and RANAP are transparently transferred, and the MSC 54/MGW 52, the HNB 30, and the UE 20 are adapted to use the same codec.